Abstract In this work, Eucalyptus Capsule Fibers (ECF) are proposed as a new natural fiber reinforcement to produce bio-composites due to their biological origin, specific smell and color. High Density Polyethylene (HDPE) is used as the matrix to compare three reinforcement types, raw ECF, alkali treated ECF, and ECF treated with PE-graft-maleic anhydride (PE-g-MA) as a coupling agent at three concentra-tions (5 wt.%, 10 wt%, and 15 wt%). A complete set of characterization is performed including tension, torsion, hardness, Melt Flow Index (MFI), Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR), Contact Angle (CA), Scanning Electron Mi-croscopy (SEM), Thermogravimetric Analysis (TGA) and Dynamic Mechanical Analysis (DMA). The results show that the best me-chanical and rheological improvements are obtained by using the coupling agent with alkali treated fibers.